1. Field of the Invention
The present invention generally relates to an electrical energy storage device, and more particularly, to a substrate provided with a thermal spray generated pseudocapacitive coating for incorporation into an electrical energy storage device. Still more particularly, the present invention provides a thermal spray generated porous, high surface area metal oxide, metal nitride, metal carbon nitride or metal carbide coating on a conductive foil for use in a capacitor, an electrochemical cell and the like. Thermal spray processes include both chemical combustion spraying and electric heating spraying, using both wire and powder forms of active materials.
2. Prior Art
In redox active structures, energy storage occurs during a change in the oxidation state of the metal when an ionic species from a conducting electrolyte, for example a proton, reacts with the surface or bulk of the oxide. This chemisorption is accompanied by the simultaneous incorporation of an electron into the oxide. The surface (or bulk) interaction between the electrode and electrolyte gives rise to capacitance in the hundreds of xcexcF/sq.cm. It follows that an electrode with high specific surface area will store a significant amount of energy and will have a large specific capacitance. These electrodes are then appropriate when used as the anode and/or cathode in electrochemical capacitors or as cathodes in electrolytic capacitors, which require high specific capacitances.
Whether an anode or a cathode in an electrochemical capacitor or the cathode in an electrolytic capacitor, a capacitor electrode generally includes a substrate of a conductive metal such as titanium or tantalum provided with a semiconductive or pseudocapacitive oxide coating, nitride coating, carbon nitride coating, or carbide coating. The prior art describes various methods of contacting the substrate with those types of active materials. Commonly used techniques include dipping and pressurized air atomization spraying of the active material onto the substrate. Capacitance values for electrodes made by dipping, pressurized air atomization spraying and sputtering are lower in specific capacitance. Sol-gel deposition is another conventional method of coating the substrate. Additionally, it is exceptionally difficult to accurately control the coating morphology due to the controllability and repeatability of the various prior art techniques, which directly impacts capacitance.
Therefore, while electrochemical capacitors provide much higher energy storage densities than conventional capacitors, there is a need to further increase the energy storage capacity of such devices. One way of accomplishing this is to provide electrodes which can be manufactured with repeatably controllable morphology according to the present invention, in turn benefiting repeatably increased effective surface areas. In the case of a ruthenium oxide cathode, the coating according to the present invention is formed by thermal spraying a highly porous, high surface area pseudocapacitive film of ruthenium oxide onto the substrate.
The present invention, therefore, describes the thermal spray deposition of a metal-containing pseudocapacitive active material onto a conductive substrate. Thermal spraying is defined as any process in which particles are heated to a molten or plastic state and propelled onto a substrate to form a coating thereon. This includes chemical combustion spraying and electrical heating spraying, using both wire and powder processes. Thus, the present invention includes heating and projecting particles of an active material onto a prepared electrode substrate using a thermal spraying process to thereby provide an electrode component for an energy storage device such as a capacitor or an electrochemical cell and the like.
Thermal spray coatings of electrode active material usually exhibit two common features in the as-deposited condition, a sandpaper-like surface finish and a structure with inherent porosity. In that respect, thermal spray coating a pseudocapacitive active material onto a substrate provides a relatively low density, porous structure. This advantageously benefits the specific surface area which in turn benefits energy storage capacity and energy storage density and ultimately discharge efficiency. Therefore, the capacitance of pseudocapacitors can be further improved by using an electrode comprising a conductive substrate coated with a thermal spray deposited porous film to increase the surface area of the electrode.
These and other aspects of the present invention will become more apparent to those skilled in the art by reference to the following description and the appended drawings.